Electrophotography encompasses image forming technologies such as, for example, photocopying and laser printing. In these technologies a latent, electrostatic image is produced by forming an electrostatic charge on the surface of a photoconductive component (e.g. a drum) and partially or fully discharging the electrostatic charge on parts of the surface of the photoconductive component by exposing those parts to light. The exposure may be from light reflected from an illuminated image (photocopying) or from a laser which scans the photoconductive component, usually under instruction from a computer (laser printing). Once a latent image has been produced in charge it is developed, using a toner, to form a visible toner image on the photoconductive component which can then be transferred onto a suitable substrate (e.g. paper) so that a hard copy of the image is obtained after fixing the toner to the substrate. During use, friction between particles of toner, with their carrier and/or with parts of the electrophotographic apparatus cause the toner particles to become charged with an electrostatic charge (tribocharge) which enables them to develop the latent, electrostatic image. The toner may be employed without a magnetic carrier as so-called “one-component” (or “mono-component”) developer or the toner may be employed with a magnetic carrier as so-called “two component” (or “dual-component”) developer.
Toner comprises toner particles typically of average particle size 1-50 μm but more usually 2-15 μm. The toner particles typically comprise a binder resin, a colorant and optionally other ingredients such as, for example, wax, lubricant and/or charge control agent to improve the properties of the toner. The resin acts to fix the toner to the substrate, usually by fusion of the resin onto the substrate by heating. The colorant, which is usually a pigment, imparts the required colour to the toner. Toners typically also comprise one or more surface additives mixed with the toner particles to modify properties including flowability and chargeability.
Desirable characteristics of toners include being capable of forming an image with high resolution and high image density, without significant print defects such as, for example, fogging, ghosting and/or spotting. Moreover, there are many other demanding performance requirements of a toner. For instance, a toner desirably possesses as many of the following characteristics as possible: fixability to a substrate at low temperatures (e.g. by means of heated fusion rollers); releasability from fusion rollers over a wide range of fusion temperatures and/or speeds and/or over a wide range of toner print densities; good storage stability; good print transparency; good toner charging characteristics but without leading to background development of the photoconductor; avoidance of filming of a metering blade and/or development roller (for a mono-component device) or the carrier bead (for a dual-component device), or of the photoconductor; high transfer efficiency from the photoconductor to the substrate or intermediate transfer belt or roller and from the transfer belt or roller (where used) to the substrate; efficient cleaning of any residual toner remaining after image transfer where a mechanical cleaning device is used.
More recently, particular focus has been on achieving higher printing speeds (e.g. 30 pages per minute (ppm) and faster) which in turn demand higher fusion speeds of the toner to the substrate. Accordingly, there is especially a need for toners which are capable of good fixation at higher fusion speeds whilst still having many of the above desirable characteristics, for example resistance to filming and good storage stability.
Toners can be conventionally produced by melt kneading of a pigment, resin and other toner ingredients, followed by milling or pulverisation to produce toner sized particles. Classification is then needed to generate an acceptably narrow particle size distribution of the toner particles.
More recently, attention has been focussed on wet chemical routes to toners, where a suitable particle size is attained directly and not by a milling process, which thereby may avoid the need for a classification step. By avoiding the classification step, less material is wasted and higher yields of toner can be attained, especially as the target particle size is reduced. Lower particle size toners are of considerable interest for a number of reasons, including better print resolution, lower pile height, greater yield from a toner cartridge, faster or lower temperature fusing, and lower paper curl.
Several wet chemical routes to toners have been exemplified in the prior art. These include suspension polymerisation, solution-dispersion processes and so-called aggregation processes. Aggregation processes may provide good control over toner size and shape amongst other features and allow for efficient incorporation of different components in the toner. Several different types of aggregation processes are known, for example, as described in U.S. Pat. No. 4,996,127, U.S. Pat. No. 5,418,108, U.S. Pat. No. 5,066,560, U.S. Pat. No. 4,983,488 and PCT patent application WO 98/50828. In aggregation processes, typically, dispersed resin particles (and preferably colorant particles and optionally particles of other ingredients such as a release agent) in a liquid medium are associated to form larger, aggregate particles, which are useful as toner particles, optionally after further treatment such as heat treatment to fuse and/or shape the aggregate particles. Further such processes are described in U.S. Pat. No. 5,344,738, U.S. Pat. No. 5,364,729, U.S. Pat. No. 5,366,841, U.S. Pat. No. 5,370,963, U.S. Pat. No. 5,370,964, U.S. Pat. No. 5,391,456, U.S. Pat. No. 5,403,693, U.S. Pat. No. 5,405,728, U.S. Pat. No. 5,910,389, U.S. Pat. No. 6,096,465, U.S. Pat. No. 6,214,510, U.S. Pat. No. 6,251,556, U.S. Pat. No. 6,828,073, U.S. Pat. No. 6,838,220, U.S. Pat. No. 6,846,603, U.S. Pat. No. 7,001,701 and U.S. Pat. No. 7,018,764.
However, it remains desirable to provide further toners and processes for making toners in which one or more of the above mentioned properties are improved.